1. Field of the Invention
The present invention relates generally to power amplifiers and, more particularly, to a method and apparatus for linearizing a power amplifier.
2. Discussion of the Background Art
Competition has led to rapid and sweeping innovations in cellular telephone technology. Analog cellular systems are now competing with digital cellular systems. In order to maximize the number of subscribers that can be serviced in a single cellular system, frequency reuse is maximized by making individual cell sites smaller and using a greater number of cell sites to cover the same geographical area. Accordingly, the increased number of cellular base stations has resulted in increased infrastructure costs. To offset this increased cost, cellular service providers are eager to implement any innovations that may reduce equipment costs, maintenance and repair costs, and operating costs, or that may increase service quality and reliability, as well the number of subscribers that the cellular system can service.
Much of this innovation has focused on service quality improvements, such as expanded digital PCS services or smaller and lighter cellular phone handsets having a longer battery life. In pursuit of the latter objective, an appreciation has developed for ways to improve the efficiency of the RF power amplifier which is used to amplify the RF signal to a level suitable for transmission within the network. The efficiency of an RF power amplifier has a significant impact on the battery life of a portable device, such as a portable transmitter, because the amplifier typically consumes the most amount of power used by the device. Efficient power amplifiers are therefore highly desirable for portable transmitters. Efficient class C, D, E, and F power amplifiers are only capable of generating constant-amplitude outputs. However, many recent transmitter designs require a non-constant amplitude RF output to maximize the data rate within a given channel bandwidth.
A suitable linear RF amplifier may be manufactured using gallium arsenide devices. However, gallium arsenide devices are presently considered too expensive for many applications. While MOS is the preferred process for manufacturing semiconductor devices, due to its low cost of fabrication and high yields, MOS has traditionally been unsuitable for fabricating linear RF amplifiers due to its lack of linearity when used to implement a high efficiency amplifier. Such poor linearity introduces a significant amount of distortion into the amplifier""s output signal. Many different linearization schemes have been proposed in the art to achieve a linear and efficient power amplifier.
The design of traditional linear power amplifiers normally involves a trade-off between efficiency and linearity. Polar modulation is a technique known in the art that simultaneously achieves linearity and efficiency in an RF power amplifier. Polar modulation is also known as envelope elimination and restoration (EER). In this approach, an RF input signal is decomposed into its polar components, i.e., phase and magnitude. These two polar components are amplified independently and are then recombined to generate an amplified, linear RF output signal. The phase component of the RF input signal is typically amplified by a constant-amplitude amplifier that is optimized for efficiency. The magnitude or envelope component of the RF input signal is typically amplified by a switching-mode power supply that operates as the power supply for at least the output stage of the constant-amplitude amplifier.
Various approaches to the use of polar modulation have been described by L. Kahn, Single-Sided Transmission by Envelope Elimination and Restoration, Proc. IRE, July 1952, pp. 803-806; and by M. Koch, R. Fisher, a High-Frequency 835 MHz Linear Power Amplifier for Digital Cellular Telephony, 39th IEEE Vehicular Technology Conference, 3 May 1989. FIG. 1 is a block schematic diagram of a traditional RF amplifier 10 that employs the above-described envelope elimination and restoration technique. In the amplifier shown in FIG. 1, an RF input signal 12 is first decomposed into its polar components. These polar components comprise phase, which is a constant-amplitude signal, and magnitude, which is a low-frequency envelope signal. The phase and magnitude components are amplified independently along separate paths 15 and 11, respectively. The phase and magnitude components are then recombined to generate the linearly-amplified RF output signal 19.
The phase component is extracted from the RF input signal by the limiter 16, and is amplified by an efficient constant-amplitude amplifier that may comprise the non-linear preamplifier 17, and the efficient, non-linear phase output stage 18. The magnitude component, which has a bandwidth comparable with the channel bandwidth, is extracted from the RF input signal by the envelope detector 13, and is amplified by the linear baseband amplifier 14. To maximize efficiency, the linear baseband amplifier 14 is implemented using a switching-mode power supply having a class-D amplifier as its output stage.
Existing implementations of switching-mode power supplies use pulse width modulation. The output of such a power supply is a square wave whose mark/space ratio represents the magnitude component of the RF input signal. However, using pulse width modulation to amplify the magnitude component introduces intermodulation distortion into the RF output. It is therefore desirable to provide a high efficiency RF amplifier that can be fabricated using a low-cost process, such as MOS, and that provides linear amplification of the RF input signal.
The aforementioned objective is achieved, and an advance is made in the art, by a transmitter for directly modulating an RF carrier with a complex baseband waveform that comprises a programmable device adapted to generate amplitude and phase information collectively corresponding to a complex baseband waveform, a signal generator adapted to generate an RF carrier whose phase is responsive to phase information generated by the programmable device, a plurality of delta modulators, and an amplifier coupled to the signal generator and receiving the RF carrier with phase information. Each delta modulator is operative to sample an error signal at a time period offset from all other delta modulators to thereby generate streams of pulses collectively approximating the amplitude information. The supply voltage of the amplifier is adjusted in accordance with approximated amplitude information represented by the streams of pulses generated by the delta modulators. Specifically, the supply voltage is adjusted such that the output voltage of the amplifier varies substantially linearly with changes in the supply voltage. In this manner, both the amplitude information and the phase information are both impressed upon the RF carrier.
A method of operating a transmitter to transmit a complex baseband waveform modulated onto an RF carrier comprises the steps of receiving amplitude information derived from the complex baseband waveform, receiving an RF carrier bearing phase information derived from the complex baseband waveform, and sampling, with a plurality of delta modulators, an error signal with each delta modulator sampling the error signal at a time period offset from all the other modulators to thereby generate streams of pulses collectively approximating the amplitude information. The method further comprises a step of receiving the RF carrier with phase information at an amplifier, and adjusting a supply voltage of the amplifier in accordance with the approximated amplitude information represented by the streams of pulses output by the delta modulators such that the output voltage of the amplifier varies substantially linearly with changes in the supply voltage, whereby the amplitude information and phase information are both impressed upon the RF carrier.